In LTE-Advanced under standardization developed by the 3GPP, carrier aggregation is now being studied in which a transmission band is divided into component carriers (CCs), uplink or downlink packets are transmitted every CC, and some coordination is made among CCs. A downlink component carrier may be defined as a hand obtained by division of a frequency band based on downlink frequency band information in a BCH broadcasted from a base station or as a band defined by a discrete width in the case where a downlink control channel (PDCCH) is discretely allocated in the frequency domain. An uplink component carrier may be defined as a band divided by uplink frequency band information in a BCH broadcasted from a base station or as a base unit of a communication band of 20 MHz or less having a physical uplink shared channel (PUSCH) domain in the center and having pieces of a PUCCH for LTE at the both ends.
The component carrier may be defined by a physical cell number or a carrier frequency number, and is also called a cell.
LTE and LTE-Advanced employ single carrier (SC)-FDMA as an uplink transmission scheme. In SC-FDMA, a transmission waveform by an individual user has single-carrier characteristics. For this reason, SC-FDMA can maintain cubic metric (CM)/PARP low. Furthermore, SC-FDMA has characteristics of orthogonal frequency division multiplexing. SC-FDMA thus can employ frequency division multiplexing (FDM) as a multiplexing method to frequency-division multiplex pieces of data in adjacent subcarriers at the same time, the data being to be transmitted to a plurality of users.
LTE and LTE-Advanced employ hybrid automatic repeat request (H-ARQ) to improve the efficiency of the transmission rate of downlink packets. When H-ARQ is employed in transmission of downlink packets, an uplink control channel (PUCCH) is used as a response channel for transmitting from a terminal to a base station an ACK or a NACK that indicates whether a downlink packet is successfully received or not received as a feedback.
In LTE in which communication is made using CC pairs each formed of an uplink CC and a downlink CC, downlink packets are transmitted in one downlink CC and a response signal is transmitted in a PUCCH of an uplink CC corresponding to the downlink CC.
When this scheme for LTE is applied to LTE-Advanced in which downlink packets are concurrently transmitted using a plurality of downlink CCs, response signals are transmitted using a plurality of uplink CCs. Transmitting a response signal by SC-FDMA in this situation causes the single-carrier characteristics of SC-FDMA to be lost. For this reason, cubic metric (CM)/peak-to-average power ratio (PAPR) increases (see FIG. 1), consequently causing the transmission waveform to be distorted. To prevent this distortion, it may be possible to cut off an amount of transmission power to be amplified, which, however impairs required reception quality.
To overcome this problem, i.e. to prevent the response signals from being transmitted concurrently in PUCCHs of a plurality of CCs, a channel selection technique has been suggested. The channel selection is a technique in which one response signal is transmitted by changing a combination of an uplink CC and constellation points (points on the constellation) according to a pattern about successful reception of each of downlink packets that have been transmitted using a plurality of downlink CCs, the uplink CCs and constellation points being used to transmit a response signal (see Non-Patent Literature 1). That is, information about ACKs or NACKs for a plurality of transmitted downlink packets are combined and one response signal is transmitted as a feedback in a PUCCH of one CC (see FIGS. 2 and 3). Here, “CC#1 DTX” in FIG. 3 refers to the case where a terminal fails to receive downlink control information for reporting to the terminal that DL#1 has been transmitted (i.e. downlink control information required for the terminal to receive DL#1 and is transmitted prior to DL#0).